This invention relates to an integrated circuit (which may be abbreviated to "IC") for use in controlling a flexible or floppy disk drive (which may be abbreviated to "FDD") for driving a flexible or floppy disk (which may be abbreviated to "FD") loaded therein.
As is well known in the art, the flexible disk drive of the type is a device for carrying out data recording and reproducing operation to and from the magnetic recording medium of the flexible disk loaded therein. In addition, such a flexible disk drive is loaded in a portable electronic equipment such as a laptop personal computer, a notebook-size personal computer, a notebook-size word processor, or the like.
The flexible disk drive of the type comprises a magnetic head for reading/writing data from/to the magnetic recording medium of the flexible disk, a carriage assembly for supporting the magnetic head at a tip thereof with the magnetic head movably along a predetermined radial direction to the flexible disk, a stepping motor for moving the carriage assembly along the predetermined radial direction, and a spindle motor for rotatably driving the magnetic recording medium with the flexible disk held.
In order to control such a flexible disk drive, an FDD control apparatus is already proposed. By way of example, Japanese Unexamined Patent Publication Tokkai No. Hei 9-97,493 (97,493/1997) discloses, as the FDD control apparatus, one integrated circuit (IC) chip which incorporates first through third control circuits therein. The first control circuit is a reading/writing (hereinafter called "R/W") control circuit for controlling reading/writing of data. The second control circuit is a stepping motor (hereinafter called "STP") control circuit for controlling drive of the stepping motor. The third control circuit is a general controlling (hereinafter called "CTL") control circuit for controlling whole operation of the flexible disk drive. The CTL control circuit may be called a logic circuit. This IC chip is generally implemented by a metal oxide semiconductor (MOS) IC chip where a number of MOS field effect transistors (FETs) are integrated therin.
The FDD control apparatus comprises not only the one IC chip but also a spindle motor IC chip for controlling drive of the spindle motor. The spindle motor IC chip is implemented by a bipolar chip where a number of bipolar transistors are integrated therin.
Now, flexible disk drives have different specifications due to customers or users. The specification defines, for example, drive select 0 or 1, the presence or absence of a special seek function, the presence or absence of an automatic chucking function, a difference of logic for a density out signal, a difference of logic for a mode select signal, 1M mode 250 kbps or 300 kbps, and so on. If development is made of one IC chips which individually satisfy the different specification, a number types of one IC chips must be prepared. In order to avoid this, a one IC chip having a selectable function circuit is already proposed, for example, in Japanese Unexamined Patent Publication Tokkai No. Hei 9-97,839 (97,839/1997) wherein all functions satisfying all specifications are preliminarily incorporated therein and one of the functions is selected in accordance with a particular specification.
As is well known in the art, the flexible disk driven by the flexible disk drive includes a disk-shaped magnetic recording medium accessed by the magnetic head. The magnetic recording medium has a plurality of tracks on a surface thereof that serve as paths for recording data and that are formed in a concentric circle along a radial direction. The flexible disk has eighty tracks on side which include the most outer circumference track (which is named "TR00") and the most inner circumference track (which is named "TR79"). The most outer circumference track TR00 is herein called the most end track.
It is necessary to position the magnetic head at a desired track position in a case where the flexible disk is accessed by the magnetic head in the flexible disk drive. For this purpose, the carriage assembly for supporting the magnetic head at the tip thereof must be positioned. Inasmuch as the stepping motor is used as a driving arrangement for driving the carriage assembly, it is possible to easily carry out the positioning of the carriage assembly. In spite of this, it is necessary for the flexile disk drive to detect only the position of the most end track TR00 in the magnetic recording medium of the flexible disk loaded therein. In order to detect the position of the most end track TR00, the carriage assembly is provided with an interception plate which projects from a base section thereof downwards and a photointerrupter is mounted on a substrate in the vicinity of a main frame opposed to the carriage assembly. For example, see Japanese Unexamined Patent Publication Tokkai No. Hei 9-91,859 (91,859/1997). That is, it is possible to detect that the magnetic head is laid in the position of the most end track TR00 in the magnetic recording medium of the flexible disk because the interception plate intercepts an optical path in the photointerrupter. Such a track position detecting mechanism is called a 00 sensor in the art.
Now, the description will proceed to a conventional FDD integrated circuit including a selectable function circuit. As described above, the FDD integrated circuit comprises the R/W control circuit, the STP control circuit, the CTL control circuit (the logic circuit), and the selectable function circuit.
The selectable function circuit comprises a selectable function terminal, a resistive potential divider, first and second comparators, and a logical circuit (a decision circuit).
In the conventional FDD integrated circuit, output signals of the selectable function circuit are directly supplied to the logic circuit as it is. As is well known in the art, immediately after a power supply is applied, a power supply voltage changes and is unstable. Accordingly, immediately after applying of the power supply, the first and the second comparators may produce erroneous first and second function selected input levels. As a result, the logical or decision circuit may carry out erroneous decision. Therefore, the CTL control circuit (logic circuit) may erroneously recognize a function which is different from a set level set by the selectable function terminal. On the other hand, the power supply voltage may change due to any cause during operation of the flexible disk drive. Under the circumstances, the logic circuit may erroneously recognize an erroneous function.